The present invention relates to adapting an automated transmission of a heavy commercial vehicle in view of the active state of an associated power take off unit that should not be driven at a speed exceeding a certain specified and safe limit.
Heavy commercial vehicles such as overland trucks and buses are known to employ automatic mechanical transmissions that are based on preprogrammed routines. Automatic mechanical transmissions of the stage-geared gearbox type have become increasingly common in heavy-duty vehicles as microcomputer technology has continued to advance. These computer-based advancements have made it possible, with a control computer and a number of control elements, for example servo motors, to precision-control the engine speed, the connection and disconnection of an automated clutch between the engine and gearbox, and between coupling members of the gearbox relative to one another, so that smooth gearshifts are always obtained at correct engine speeds.
The advantage of this type of automatic gearbox, compared to a traditional automatic gearbox constructed with planetary gear steps and having a hydrodynamic torque converter on the input side, is two-fold. In a first aspect, and particularly with regard to use in heavy vehicles, the electronic based AMT system is more simple and robust, and can be produced at substantially lower cost than traditional automated gearboxes. In a second aspect, the electronic based AMT system enables the vehicle to be more efficient with respect to fuel consumption.
One of the problems in controlling an AMT, however, is attributable to the power consumption imposed by an associated power take off (PTO). A PTO can generally be classified as a PTO upstream or downstream of the master clutch. In general, a PTO that is upstream of the master clutch can take power from the vehicle's engine regardless of the state of engagement and activity of the vehicle's transmission via the master clutch. In contrast, a PTO that is located downstream of the master clutch is typically used only when the vehicle is stationary. Typically, a downstream PTO often involves placing the gearbox in neutral so that the vehicle wheels are not drivingly engaged to the transmission. However, there are cases when a downstream, transmission mounted PTO is used while the vehicle is in motion.
PTOs are known to impose significant load on the vehicle's engine. Exemplary PTOs use engine power to drive hydraulic pumps that can be activated for such things as mixing applications (concrete trucks) or causing motion of a bed on the truck such as in the case of dump trucks and flat-bed haulers. Similarly, PTOs may be used to power spreaders such as those used to broadcast salt or sand on icy roads, or to power associated trailer components such as compartment refrigeration units. While these examples are not exhaustive, they do serve to exemplify PTO loads of significant magnitude which can appreciably compromise the driving power available from the engine of the vehicle for the drive wheels, and which often causes undesirable disturbances to automated transmission programs that do not take their intermittent influences into account. For purposes of comparison, these significant PTO loads can be compared to less influential engine loads imposed by such power consumers as cooling fans and air conditioning compressors. As an example of the potential drag that a PTO can impose on the vehicle's engine, it is not uncommon for PTOs to siphon off engine torque on the order of 5 to 3000 Nm. An example of a PTO that requires on the order of 3000 Nm is a fire truck that operates a water pump, and an example of a PTO that requires on the order of 5 Nm would correspond to a PTO for powering a small refrigeration device.
Also, a PTO can be limited in its ability to handle excessive engine speeds. An example of this might be that of a cement mixer where the PTO is limited to 1500 rotations per minute (rpm). Other known PTOs have speed limits between 1300 and 2000 rpm. The danger of exceeding the speed limit of the PTO is that it can lead to equipment failure, which could have significant safety risks associated therewith, besides damaging the equipment. In order to prevent this problem from occurring, the engine speed, while the PTO is engaged, should remain below a corresponding limit. Typical maximum operating conditions for a diesel engine exist at or around 2100 rpm. In certain circumstances, however, diesel engines can approach speeds on the order of 2400 rpm. These circumstances are usually only allowed for short periods such as during engine braking, for example. Other engines may have speed limits higher than that of a diesel engine and could be on the order of 5000 rpm. The referenced engine may serve as an example of a more general prime mover of the vehicle, a term which encompasses any power plant that provides power to the driveline of the vehicle or other power consuming components of the vehicle. Examples of such prime movers are diesel engines, electric motors, and hybrid power systems.
The present invention appreciates the fact that current transmission control routines typically do not take into consideration engine speeds which might cause damage to speed sensitive PTO loads. If the PTO becomes at risk of failure above a certain speed and the transmission control unit allows the engine to increase speed above that point, serious damage is possible to the directly engine connected PTO. This situation can occur where the transmission control unit requests a shift that requires the engine speed to exceed the rated speed of the PTO.
Still further, it has been appreciated that it can be difficult to adjust a transmission when the limits of an associated PTO are about to be exceeded. Therefore, one aspect of the present invention has as a goal to provide a solution where no shift or other transmission function is made that requires the engine to exceed the speed allowed by the PTO when engaged. Such a function is particularly important when it is appreciated that the power-drag of the PTO often causes the vehicle to operate in lower gears (higher gear ratios) with correspondingly higher engine speeds which can damage the PTO if not controlled.
In at least one embodiment, the present invention takes the form of a method for limiting the engine speed to below a maximum allowable PTO operating speed. The method comprises (includes, but is not necessarily limited to) using a speed limited PTO to set a maximum allowable engine speed. Before processing a gear change, the transmission control unit ensures that the corresponding engine speed never exceeds the specified PTO maximum speed. If the PTO is disengaged, then the transmission control unit will allow shifting without regard to the limited PTO speed. Thus, the invention is active when there is a PTO engaged and that the PTO has a speed limit.
The speed limitations are entered into the transmission control unit or another control unit including, but not limited to those described below and stored for a given PTO. It is contemplated that the stored limit(s) can be changed, and that the transmission control unit will sense (or receive information regarding) when the PTO is engaged. Alternatively, the information might be stored in one control unit and communicated to the other control unit. The vehicle control unit might store the value of the PTO speed limit, and when a shift request is made, a request to determine that limit is made by the transmission controller and the vehicle control sends the appropriate signal to the transmission controller.
Additionally, the engine speed is also controlled through the transmission control unit to ensure that the engine speed does not exceed the PTO speed limit. This may be used in combination with the transmission gear shifting limits, or standing alone.